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Time-domain analysis, like standard deviation and root mean square (RMS), provides powerful differentiators of stressed versus relaxed conditions. Some examples include SDNN (the standard deviation of NN (or R-R) intervals), which provides a measure of changes in heart rate stemming from cycles longer than five minutes, and NN50, which notes the number of pairs of successive NN intervals that differ by more than 50 msec. Frequency domain analysis shows the ratio of parasympathetic and sympathetic activity. High frequency pertains to the parasympathetic system and the vagus nerve, which controls the parasympathetic nervous system Low frequency pertains to sympathetic activity Very low frequency pertains to the sympathetic nervous system, chemoreceptors, thermoreceptors, and the renin-angiotensin system (i.e., hormones) Non-linear analysis can point to underlying cardiac conditions and includes: Detrended fluctuation analysis (DFA), which looks for self-similar patterns by analyzing the power spectral density (PSD). Peaks in PSD indicate repetitive patterns. Entropy analysis, a measure of randomness over time. Decreased HRV and increased randomness of HR are independently associated with high-risk conditions. Poincaré plot analysis utilizes scatter plots of consecutive pulse interval points. Consecutive pulses that vary by large amounts will have larger scattering around the diagonal. Low HRV will shrink and cluster around the diagonal. Unbalanced HR behaviors such as fast acceleration and slow deceleration will generate asymmetric plots. Large off-diagonals show skipped heartbeats, which usually indicate an arrhythmia problem.
DESIGN WORLD — EE NETWORK
Role of HRV FITNESS & ENDURANCE
BEAT TO BEAT HR
HRV
STRESS & RECOVERY
CARDIAC HEALTH HRV provides an indicator of potential cardiac conditions. UNDERSTANDING EXERCISE METRICS Let’s now take a look at exercise physiology and associated metrics. According to the American Heart Association, cardiorespiratory fitness is a better indicator of mortality than any other risk factors, including smoking, hypertension, and high cholesterol. The current gold standard to determine cardiorespiratory fitness is the VO2 max test, which tests for the maximum amount of oxygen that one can use during intense exercise. VO2 max is related to peak endurance. The test is generally administered in a medical facility using a treadmill and an oxygen mask. Other important exercise metrics include measurements of HR and recovery and of excess post-exercise oxygen consumption (EPOC). Based on the energy expended during exercise, HR goes up to deliver the oxygen needed for energy production. Max HR is the upper limit. The max HR scale is divided into five zones, based on exercise strength or the exercise goal. The target heart rates are agedependent. Zone 1 is the very low intensity zone (50-60% of max HR). Training at this intensity will boost recovery and get you ready for higher zones. Zone 2 is essential for a runner’s program (60-70% of max HR). Exercising at this zone will improve general endurance. The last three zones, however, are the most interesting: Zone 3 is the aerobic level (70-80% of max HR). This is a comfort zone, where oxygen is readily available, and energy is generated mostly from burning fat. Zone 4 is the anaerobic level (80-90% of max HR). This is out of the comfort zone, where the goal is to hit the maximum oxygen consumption 6 • 2020
(VO2 max) and remain there as long as possible to continuing burning fat post-exercise. Zone 5 (more than 90% of max HR) is not recommended, as this is not at a healthy level and can trigger long-term health effects. Recovery time after exercise depends on the total O2 deficit. EPOC refers to the amount of extra oxygen needed to recover after exercise. This extra oxygen consumption actually burns additional calories after exercise. The recovery itself has a fast and a slow component. During the fast component, the muscles return back to their normal state. During the slow component, the lactic acid that is generated during exercise (as glucose is burned to generate energy) is removed from the muscles. EPOC lasts up to 48 hours depending on duration and intensity of the exercise. Besides the gold-standard VO2 max test, there are other well-defined protocols that can guide exercise. Some are geared toward establishing optimal training processes, while others are for improving the oxygen consumption rate and still others are aimed at increasing post-exercise energy consumption. During the exercise, VO2 increases as the workload increases. However, at some point, despite an increase in workload, an individual will reach his or her VO2 max. VO2 can be improved with training. Crosscountry skiers, runners, and swimmers tend to have the largest VO2 max.
HRV-GUIDED TRAINING According to Market Reports Hub, the smart sports and fitness wearables market is projected eeworldonline.com
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